One of my customers was a production machine shop,and one thing they used to do was make guns for our movie industry - most were just props,but some had to be fired as well. They had a nice gattling gun out the back of the shop that fired .22's they used to test fire sometimes.
One day the owner and I were going home in the same traffic jam,I was in my Pajero,he in a Rangerover,so we were higher than the cars around us. He held his hand palm out towards me,and concealed in his palm was a small gun....either something for a job,or just one he made for himself. Small hand grafted weapons are certainly out there.